Journeys to the Center of the Earth | DiscoverMagazine.com Journeys to the Center of the Earth FROM THE JULY/AUGUST 2014 ISSUE Journeys to the Center of the Earth Our planet's core powers a magnetic field that shields us from a hostile cosmos. But how does it really work? By Tim Folger |Monday, July 14, 2014 RELATED TAGS: EARTH SCIENCE Roen Kelly On the remote Kola Peninsula in northwestern Russia, amid the rusting ruins of an abandoned scientific research station, is the deepest hole in the world. Now covered and sealed with a welded metal plate, the Kola Superdeep Borehole, as it’s called, is a remnant of a largely forgotten Cold War race that aimed not at the stars, but at Earth’s interior. A team of Soviet scientists began drilling at Kola in the spring of 1970, with the goal of penetrating as far into Earth’s crust as their technology would allow.

Four years before the Russians started punching their way into the Kola crust, the United States had given up on its own deep-drilling program: Project Mohole, an attempt to bore several miles through the Pacific seafloor and retrieve a sample of the underlying mantle. Mohole fell far short of its target, reaching a depth of just 601 feet after five years of drilling under more than 11,000 feet of water. The Soviets were more persistent. Their work at Kola continued for 24 years — the project outlived the Soviet Union itself. Before drilling ended in 1994, the team hit a layer of 2.7-billion-year-old rock, almost a billion years older than the Vishnu schist at the base of the Grand Canyon. Temperatures at the bottom of the Kola hole exceeded 300 degrees Fahrenheit; the rocks were so plastic that the hole started to close whenever the drill was withdrawn. While the researchers at Kola bored patiently downward, their counterparts in the space race sent dozens of craft heavenward: as far as the moon, Mars and beyond.

By the early 1990s, when the Kola effort began to stall, the Voyager spacecraft had already passed beyond Pluto’s orbit. And the depth of the Kola hole after 24 years of drilling? About 7.6 miles — deeper than an inverted Mount Everest and roughly halfway to the mantle, but still a minuscule distance, considering Earth’s 7,918-mile diameter. If Earth were the size of an apple, the Kola hole wouldn’t even break through the skin. Illustration: Roen Kelly, Photo: A. Varfolomeeviria Novosti All the mines on Earth, all the tunnels, caves and chasms, all the seas, and all of life exist within or on top of the thin shell of our planet’s rocky crust, which is much thinner, comparatively, than an eggshell. Earth’s immense, deep interior — the mantle and core — has never been directly explored, and probably never will be. Everything we know about the mantle, which begins about 15 miles below the surface, and about Earth’s core, 1,800 miles beneath us, has been gleaned remotely.

While our understanding of the rest of the universe grows almost daily, knowledge of the inner workings of our own world advances far more slowly. “Going into space is just a lot easier than going down for an equivalent distance,” says David Stevenson, a geophysicist at the California Institute of Technology. “Going down from 5 kilometers to 10 is much harder than going from zero to 5.” What scientists do know is that life on Earth’s surface is profoundly affected by what happens at inaccessible depths. Heat from Earth’s inner core, which is as hot as the surface of the sun, churns an outer core of molten iron and nickel, generating a magnetic field that deflects lethal cosmic and solar radiation away from the planet. For a glimpse of what Earth might be like without its protective magnetic shield, we have only to look at the lifeless surfaces of worlds with anemic magnetic fields, like Mars and Venus.

For a glimpse of what Earth might be like without its protective magnetic shield, we have only to look at the lifeless surface of a world like Venus The planetary architecture that provides Earth’s sheltering field has been broadly understood for several decades now: a solid-iron inner core roughly the size of the moon, surrounded by a 1,400-mile-thick outer core of liquid iron and nickel, with 1,800 miles of solid mantle above, topped by a crust of slowly drifting tectonic plates. But when it comes to the very center of the planet, this blueprint is sorely incomplete. “Right at this moment, there is a problem with our understanding of Earth’s core,” says Stevenson, “and it’s something that’s emerged only over the last year or two. The problem is a serious one. We do not understand how the Earth’s magnetic field has lasted for billions of years. We know that the Earth has had a magnetic field for most of its history. We don’t know how the Earth did that.

… We have less of an understanding now than we thought we had a decade ago of how the Earth’s core has operated throughout history.” The surface of Venus, shown in this artist's interpretation, appears as a bleak hellscape filled with choking sulfur clouds. The planet's magnetic field offers little protection against the sun's deadly rays. ESA/C. Carreau A Modest Proposal On a warm summer morning, I met with Stevenson at his Caltech office in Pasadena. He was dressed for the weather, wearing shorts, sandals and a short-sleeved shirt. We talked for a while about how the surfaces of Mars and other planets, despite being tens or hundreds of millions of miles away, are far more accessible than Earth’s core. “Of course, the universe above the Earth is mostly transparent! So you have the wonderful opportunity to use photons to tell you about the rest of the universe,” he says. “But you can’t do that inside the Earth. So the methods we have for seeing inside the Earth, if you will, are actually quite limited.” Eleven years ago, Stevenson published a paper in the journal Nature outlining a wild scheme to get around some of those limitations.

His article, “Mission to Earth’s core — a modest proposal,” described a way to send a small probe directly to the center of the Earth. The article’s title was a nod to Jonathan Swift’s 1729 satirical essay, “A Modest Proposal,” which mocked harsh British policies in Ireland by suggesting that the Irish alleviate their poverty by selling their children as meat to the English gentry. Like Swift, Stevenson wasn’t arguing for the actual feasibility of his idea; the paper was a thought experiment, an exercise to show the literally earthshaking scale of effort that would be needed to probe deep into the planet. The first step in Stevenson’s journey to the center of the Earth: Detonate a thermonuclear weapon to blast a crack several hundred meters deep in Earth’s surface. Next, pour 110,000 tons of molten iron into the crack. (Stevenson told me that he now thinks 110,000 tons is an underestimate. On the plus side, a nuclear explosion might not be necessary — a million tons of conventional explosives might suffice.) Molten iron, being about twice as dense as the surrounding mantle, would propagate the crack downward, all the way to the core.

The crack behind the descending blob of iron would quickly seal itself under pressure from the surrounding rock, so there would be no risk of the crack spreading catastrophically and splitting the planet wide open. Carried along with the sinking iron would be a heat-resistant probe about the size of a football. Stevenson estimated that the molten iron and probe would move at a rate of about 10 mph and reach the core in a week. The probe would record data on the temperature, pressure and composition of the rock it passed through. Since radio waves can’t penetrate solid rock, the probe would vibrate, transmitting data in a series of tiny seismic waves. An extremely sensitive seismometer on Earth’s surface would receive the signals. It’s within the reach of current technology to build a probe capable of surviving immersion in molten iron and to collect its data, but what about the rest of the plan? Could some version of Stevenson’s idea possibly work? “The particular scheme I proposed is probably impractical,” he tells me, mostly because of the enormous quantities of molten iron that would be needed. “But it was not physically ridiculous. Engineering it may have been ridiculous, but in terms of physical principles, I wasn’t violating any laws of physics.

I was showing that in a world unrestricted by concerns about how much money you would spend, you could contemplate doing what I described.” Proposing a realistic mission wasn’t the point of the paper, Stevenson says. He wanted to highlight the limits of what can be known by constructing theories about Earth’s interior from our perch on the planet’s surface. “I wanted to remind people that the history of planetary exploration has told us the importance of going there. Time after time, we have learned things when we arrive at a planet that we had not suspected by looking at that planet from afar. I believe very strongly in this aspect of science. “There is a danger that we will compartmentalize our understanding of an aspect of the universe by saying to ourselves, ‘OK, we know we can’t go there, so we’re going to build this elaborate story of what’s there based on remote observations.’ And this is what we do for the Earth,” Stevenson continues. “We don’t even know whether the material immediately adjacent to the core is entirely solid or partly solid. We don’t know the character of the core-mantle boundary.

There are a lot of questions that would only be answered with precision by going there.” The late 1600s brought fantastical views of our planet's interior such this one by Athanasius Kircher. Courtesy Bizzell Bible Collection, University of Oklahoma Libraries Seeking the Center Lacking direct access to anything beyond a few miles beneath Earth’s surface, Stevenson and other geophysicists are forced to rely on indirect methods, at least for now. Educated guesswork — and not-so-educated guesswork — has a long history in geology. While Kepler, Galileo and others were establishing the foundations of modern astronomy in the 17th century, the study of Earth itself remained a medieval science, mired in myth and fantastic imaginings. A map published in 1664 by Jesuit scholar Athanasius Kircher depicts a cavernous Earth riddled with chambers — some filled with air, some with water, some with fire. Hell occupied Earth’s blazing center; purgatory lay a bit farther out. Ducts flowing with flames warmed hot springs, fed volcanoes and tormented the damned. Whatever his faults as a theoretician, Kircher was no armchair scholar.

He once had an assistant lower him into the active and smoking crater of Mount Vesuvius so he could take temperature measurements. Even the best astronomers of the day stumbled when they turned their attentions Earthward. In a paper published in 1692, Edmond Halley, later famed for charting the orbit of his eponymous comet, argued that Earth was mostly hollow, consisting of three concentric shells rotating around a core. He estimated that the outermost shell — the one we live on — was 500 miles thick. (Halley based his calculations on a mistaken result by Isaac Newton regarding the relative masses of the moon and Earth, leading Halley to grossly underestimate Earth’s mass.) Atmospheres of glowing gas separated the shells, each of which had its own magnetic poles. Halley believed the inner shells might even be inhabited and lit by subterranean suns. The Lick Observatory in California was home to North America's first time-recording seismograph, shown here in a drawing.

Publications of the Lick Observatory, Volume I, 1887/Courtesy Lick Observatory Historical Collections A detailed picture of Earth’s structure began to emerge only after the invention of the time-recording seismograph in 1875. North America’s first seismograph was installed at Lick Observatory near San Jose, Calif., in the late 19th century; it recorded the San Francisco earthquake of 1906. By the early 20th century, a global network of the instruments allowed researchers to record seismic waves that had traveled from one side of the planet to the other. An earthquake powerful enough to be felt occurs somewhere in the world about once every 30 minutes. Each releases a variety of seismic waves. In addition to the waves that distort Earth’s surface and cause so much destruction, earthquakes spawn two other types of seismic energy that ricochet through the body of the entire planet. Primary waves, or P-waves, compress the layers of rock or liquid they pass through. They move at more than 16,000 feet per second through granite. Secondary waves, or S-waves, pull rocks apart as they undulate through the planet, creating what scientists call shear forces.

Traveling at about half the speed of P-waves, they’re the second type of wave to reach seismographs, hence their name. Secondary waves move only through solids; shear forces don’t exist in liquids (since liquids can’t be torn apart). The speeds and paths of both types of waves vary with the density and elasticity of the materials they encounter. Whenever the waves reach a boundary between regions differing in density or other properties, they are deflected from their trajectories. By analyzing these sorts of data from seismic waves, scientists can identify the rocks and metals that make up Earth’s mantle and core. Roen Kelly Until well into the 20th century, most scientists believed Earth had a liquid iron core. The evidence seemed clear: Seismic maps of Earth’s interior revealed an absence of S-waves at the center of the Earth, presumably because the waves hit a liquid zone through which they could not travel. Seismic studies also revealed that all earthquakes created a P-wave “shadow zone” on Earth’s surface where primary waves didn’t arrive at some seismic stations; the location of the P-wave shadow zone varied with the point of origin of the earthquake.

To explain the shadow zone, scientists reasoned that Earth’s presumed liquid core deflected P-waves from their expected trajectories, so they wouldn’t be recorded at all seismographic stations. Inge Lehmann SPL/Science Source The first hint that Earth actually had a solid iron core beneath a liquid layer came in 1929, after a magnitude-7.8 earthquake shook New Zealand. Such large temblors provide a wealth of data, and researchers around the world pored over seismograph recordings in the quake’s aftermath. But only one scientist noticed anything unusual. Inge Lehmann, a Danish seismologist, made meticulous notes on seismic activity, including the arrival time of P-waves, at various seismographic stations. (Lehmann kept her notes on cards that she stored in empty oatmeal boxes.) She found P-waves in what should have been P-wave shadow zones. If Earth’s core were completely liquid, P-waves should have been deflected away from the shadow zones. In a paper published in 1936, she argued that the anomalous P-waves must have been deflected from some denser structure within the liquid core, sending them on trajectories into the shadow zones.

Lehmann concluded that Earth must have a solid inner core. It wasn’t until 1970 that instruments became sensitive enough to prove beyond a doubt that she was right. Lehmann, who published her last scientific paper when she was 98, died in 1993 at the age of 104. Earth's Burning, Churning Engine With the discovery of the nature of the inner core, the basic components of Earth’s composition — and even the planet’s evolution from its molten origins — were in place. Or so it seemed until recently. New research has uncovered a flaw in our understanding of the core — specifically, about the manner in which heat energy flows from the core and through the overlying mantle. The problem raises important questions about the age of the inner core, and about how Earth generates its magnetic field, a phenomenon crucial for the existence of life. Based on the radioactive dating of ancient rocks, scientists estimate that Earth formed about 4.5 billion years ago. As the molten proto-Earth cooled, its outermost layer hardened into a thin crust. Earth’s mantle also solidified over time, though even now the temperature at the lower mantle is about 4,000 F.

The inner core, once entirely liquid, is slowly solidifying from the inside out, increasing its diameter by about half a millimeter per year, according to some estimates. Iron’s melting point is greater at higher pressure, and as the planet cooled, the extreme pressures at the very center of Earth eventually prevented the iron there from continuing to exist as a liquid. Despite sunlike temperatures, the inner core began to solidify, and it’s been growing ever since. Under slightly less pressure, the outer core — a 1,400-mile-deep, 8,000-degree ocean of iron and nickel — is still hot enough to be fluid. “It would flow through your hands like water,” says Bruce Buffett, a geophysicist at the University of California, Berkeley. All of Earth’s layers, from core to crust, are in constant motion, caused by the flow of heat. Heat moves through Earth’s interior in two fundamentally different ways: convection and conduction. Convection occurs when heat from below creates motion in the layers above — heated material rises, then falls back again as it cools, only to be heated once more. Convection is what roils a pot of boiling soup.

Deep inside Earth, slow-motion convection of rocky minerals in the mantle and heat loss from the cooling solid inner core cause convection in the liquid outer core. Roen Kelly Heat also makes its way through the Earth by conduction — the transfer of thermal energy by molecules inside a material from hotter areas to colder ones — without causing any motion. To continue the soup example, heat is conducted through the bottom of the metal pot. The metal in the pot doesn’t move; it simply transmits, or conducts, heat to the pot’s contents. The same is true inside the Earth: In addition to convection currents moving heated material through the outer core and mantle, heat is conducted through liquids and solids without roiling them. Researchers have known for many decades that the slow, convective sloshing of liquid iron in the outer core, aided by Earth’s rotation, generates the planet’s magnetic field. As the molten iron flows, it creates electric currents, which generate local magnetic fields. Those fields in turn give rise to more electric currents, an effect that results in a self-sustaining cycle called a geodynamo. Evidence from ancient rocks reveals that Earth’s geodynamo has been up and running for at least 3.5 billion years.

(When rocks form, their magnetic minerals line up with Earth’s field, and that orientation is preserved when the rocks solidify, providing geophysicists with a record, written in stone, of the planet’s magnetic past.) But here is the fundamental problem with our understanding of the geodynamo: It can’t work in the way geophysicists have long believed. Two years ago, a team of scientists from two British universities discovered that liquid iron, at the temperatures and pressures found in the outer core, conducts far more heat into the mantle than anyone had thought possible. “Earlier estimates were much too low,” says Dario Alfè, a geophysicist at University College London, who participated in the new research. “The conductivity is two or three times higher than what people used to think.” The discovery is vexing: If liquid iron conducts heat into the mantle at such a high rate, there wouldn’t be enough heat left in the outer core to churn its ocean of liquid iron. In other words, there would be no heat-driven convection in the outer core. If a pot of soup conducted heat into the surrounding air this effectively, convection would never start, and the soup would never boil.

“This is a big problem,” Alfè says, “because convection is what drives the geodynamo. We would not have a geodynamo without convection.” Alfè and his colleagues used supercomputers to carry out a “first principles” calculation of heat flow in liquid iron at Earth’s core. By first principles, they mean that they solved a set of complex equations that govern the atomic states of iron. They weren’t estimating or extrapolating from lab experiments — they were applying the laws of fundamental quantum mechanics to derive iron’s properties at extreme pressures and temperatures. The British researchers spent several years developing the mathematical techniques used in the equations; only in recent years have computers become powerful enough to solve them. “It was exciting and scary because we found values that were very different from what people have used,” Alfè says about the discovery. “The first thing you think is, ‘I don’t want to be wrong with this.’ ” No Impacts, No Magnetic Field, No Life? The work has gained wide acceptance since its publication in Nature two years ago, especially since their first principles calculations now have some experimental backing.

A team of Japanese researchers recently found that small samples of iron, when subjected to high pressures in the laboratory, displayed the same heat-transfer properties that Alfè and his colleagues predicted. Stevenson, the Caltech geophysicist, says the new values for liquid iron’s conductivity will probably stand the test of time. “It’s possible that the numbers might come down a little bit, but I’d be surprised to see them come all the way down to the conventional value,” he says. So how can the new findings be reconciled with the undeniable existence of the planet’s magnetic field? Stevenson and other researchers have previously proposed a second mechanism besides heat flow that could produce the required convection in the outer core. The inner core, although composed almost entirely of pure iron, is thought to contain traces of lighter elements, primarily oxygen and silicon. As the iron in the inner core cools and solidifies, the researchers hypothesize, some of those light elements would be squeezed out, like the salt extruded from ice crystals when seawater freezes. Those light elements would then rise into the liquid outer core, creating convection currents. This so-called compositional convection would be another way to power the geodynamo.

Much of the impact energy of primordial collisions would have been converted into heat, liquefying Earth's interior But compositional convection would work only once an inner core had already formed. In a purely liquid core, the light elements would be evenly distributed throughout the liquid, so there would be no compositional convection. Based on how fast Earth’s core is cooling and solidifying now, it’s likely that the inner core formed relatively recently, perhaps within the past billion years. How did the geodynamo manage to function for at least a couple of billion years before the inner core existed? “The problem is actually in Earth’s past,” not in the present, says Alfè. “This is where new hypotheses are coming in. Some people are saying maybe the Earth was a lot hotter in the past.” If the young Earth contained more heat than current theories account for, there might have been enough left over to power the requisite convection, even given the new findings about liquid iron’s higher conductivity. What could have provided the extra heat?

One of the leading explanations would have beggared the imaginations of even the most inventive medieval mapmakers: Primordial collisions between the young Earth and other protoplanets forced mantle material into the core, providing the heat that kick-started Earth’s geodynamo. The idea that a Mars-size body smashed into Earth roughly 4.5 billion years ago was first proposed in the 1970s, in an effort to explain the uncanny resemblance of moon rocks to terrestrial ones. Moon rocks are unique in that regard. Meteorites, for example, have chemical and elemental profiles that mark them as distinctly otherworldly. “But rocks from the moon and the Earth look identical,” says Buffett. Protoplanet collisions with a young Earth may have led to the birth of our moon and could have kick-started the geodynamo that makes life here possible.

Julian Baum/Take 27 LTD Stevenson, Buffett and other researchers suspect that the theory of the moon’s fiery birth might also solve the problem of how Earth kept its geodynamo running before the inner core formed: Much of the impact energy of primordial collisions, like the one that may have caused the moon to form, would have been converted into heat, liquefying Earth’s interior. Some of the debris from one of these impacts eventually coalesced to form the moon. Earth itself was left so hot that it would have glowed like a small star for a thousand years. “Most people think that these impacts probably set the initial thermal conditions of the planet,” says Buffett. “And the core has been tapping that heat over geological time.” Were it not for that store of excess heat, Earth’s geodynamo might never have started. And without a protective magnetic field around the planet, solar radiation would have stripped Earth’s atmosphere and bombarded the surface, which was apparently the fate of Mars. It may be that several seemingly disparate phenomena were essential in making Earth a habitable world: the formation of the moon, the planetary magnetic field, plate tectonics and the presence of water.

Without the collision that created the moon, there wouldn’t have been enough heat for convection to start in Earth’s core and power the magnetic field. Without water, Earth’s crust might have remained too strong to be broken up into tectonic plates; and without a tectonically fractured crust, too much heat would have been trapped inside Earth. Without Earth being able to cool, there would have been no convection and conduction. “Are these things connected, or are they just happy coincidences?” asks Buffett. “We don’t know for sure. These correspondences are intriguing. You can look at Venus: no plate tectonics, no water, no magnetic field. The more you look at this and think about it, the more you think it can’t be a coincidence. The thought that these things might all be connected is kind of wondrous.” Is Earth unique, then? Does life require more than oxygen, water and suitable temperatures? Are a fortuitous primordial collision and a moon also necessary, along with a churning liquid core?

How repeatable might the circumstances be that gave rise to our world, with its crust filmed with life, shielded from a hostile cosmos by a 3.5-billion-year-old internal engine of heat and iron? “It’s still not clear how unusual our solar system is,” says Stevenson. “It’s certainly clear that planets are extremely common — there is absolutely no doubt about that. But the formation of planets is not a deterministic process. It is a chaotic process that has a variety of outcomes. In our solar system alone, there are striking differences between Earth and Venus. I think it’s a matter of chance, just how the game played out, how the dice were thrown.” The answers may come as we learn more about the sorts of worlds that orbit other stars, Stevenson says. Perhaps a handful of those worlds will resemble our own, or maybe thousands will. And maybe one will have inhabitants dwelling on a thin mutable crust, drilling, monitoring tremors, building theories, seeking to understand what lies beneath them, and wondering if their world is miraculous or mundane.

ADVERTISEMENT EXPLORIT SCIENCE CENTER: Science Facts EARTH SCIENCE FACTS Introduction When the star that we call our Sun condensed out of a swirling cloud of hot interstellar gas, some material remained in orbit around the newly forming star. That orbiting material consisted of hot chunks of rock and frozen liquids called "planetisimals". From these were gradually formed several (nine that we know of) planets including Earth. The planets - including Earth - still move in relatively stable orbits around the sun. Earth not only orbits the sun but spins on its own axis as it does so. At the equator the Earth is spinning on its axis at a speed of about 1,038 miles (1,670 kilometres) an hour. Earth's spin or rotation is a relic of its origin as a hot, spinning mass when it was first formed. Earth science or geology is the science of the Earth. It is an organized study and body of knowledge about mountains, plains and oceans; about the history of life on Earth; and about the succession of physical events that have been (or are still) part of this history. 1. How was the planet Earth formed?

Chunks of rock and frozen liquids in orbit around the newly forming Sun converged and melted together to form the planets. Each time a planetisimal (see the Introduction) hit the rotating chunk of material that was to become Earth it melted in to the existing mass. The mass was hot and the metals were molten so the heavy metals like iron sank toward the center of the mass while lighter metals like silicon floated near the surface. 2. Why does the Earth have a concentric-layered structure? During the time when Earth was forming and was in a molten state, heavy materials like iron and nickel sank to the center of the Earth's mass and lighter materials floated to the surface of the mass. The very center of what we call the core, is solid in the middle because of the increased pressure in that region. The solid, central part of the core has a liquid (molten) layer surrounding it. Outside the core, the largest mass of planet Earth is called the mantle and this is mostly solid rock. The very outermost layer of our planet is about 30 miles thick, is made of lighter rocks, and is called the crust. 3. How warm is the center of Earth? The further you go toward the center of Earth the hotter it gets.

The first hundred feet below the surface is generally quite uniformly cool. However below the first hundred feet, the rocks are warmer by several degrees celsius for each 1,000 feet of depth. Very deep mines, such as deep gold mines for example, can have temperatures of as much as 130 degrees celsius. 4. How do we know what the center of Earth is like? No-one has ever penetrated Earth's mantle to reach the core to investigate what it is made of so much of what we "know" is theory based on intelligent deduction. Some deductions are based on good empirical (based on observation or experiment) data obtained by analyzing the seismic waves that are produced when earthquakes happen. Seismic waves that travel through the deep interior of the Earth can sometimes be detected by seismographs in different parts of the world. Scientists observe where these waves reach the surface and how long it takes for them to get there. Comparing these observations with controlled experiments on the behavior of waves passing through different materials, scientists can then construct good theories about what materials deep in the Earth the seismic waves passed through on their way to the surface. 5. How strong is an earthquake?

Charles Richter (1900-1985), an American seismologist, created a logarithmic scale that expresses the strength of earthquake movements. An earthquake that is only detectable by very sensitive seismographic machines is strength 2.0 to 2.9; the slight vibration and swinging of hanging objects is indication of a strength 3.0 to 3.9 earthquake. In a level 7.0 to 7.9 earthquake, buildings collapse, cracks appear in the ground and landslides occur. The Richter magnitude scale is a logarithmic scale so a level 4.0 earthquake is ten times stronger than a level 3.0. (And a level 5.0 quake is 100 times stronger than a level 3.0). The number given to the strongest earthquake described by the Richter scale is 8.9 (An 8.9 quake is more than 79 million times stronger than a 1.0 quake!) 6. How did Earth's atmosphere develop? When Earth was first formed it was a solid ball of molten rock with no atmosphere.

As Earth began to cool and the crust and mantle began to harden, material from the inner regions would sometimes erupt through the surface bringing hot gases. Scientists theorize (no-one really knows because no one was there!) that these gases were methane, ammonia, carbon dioxide and water (steam). The water eventually condensed to form oceans but almost certainly there was at first no oxygen or nitrogen in Earth's atmosphere. It is believed that the atmosphere (air) that we now breathe was formed only after the development of the first living things, blue-green algae (now generally classified as blue-green bacteria), that happened about two billion years ago. These simple organisms used carbon dioxide during photosynthesis and gave off oxygen as a biproduct of the chemical process. 7. Where did the Moon come from? The best, simple answer to this question is that we dont know.

There are several theories: a) perhaps the Moon was torn out of Earth's crust and mantle at some time in the distant past (billions of years ago); b) perhaps the Moon was created out of planetisimals (see the Introduction and Day 1) and somehow got caught by Earth's gravitational influence and went into orbit around Earth as well as around the Sun; c) perhaps a giant meteorite hit Earth soon after it was formed and the material thrown out into space as a result of the impact coalesced into a small mass that became the Moon. 8. How can we tell the age of the rocks in Earth's crust? There are several methods: a) We can make educated guesses - in general, the deeper the rock, the older it is. Using this generalization geologists have created a relative time scale that describes what is older or younger but does not give a specific ages in years except in the most general terms; b) It is also possible by comparing the fossils in two different rock layers to tell that they are about the same age if the same types of fossils are found in each; c) Radiometric dating such as the carbon-14 method can provide quite accurate specific ages of the remains of living things. 9.

How old is Earth? Earth's oldest rocks are believed to be about 3.8 or 3.9 billion years old. Because rocks formed quite slowly from the Earth's original molten mass, clearly Earth is older than its oldest rocks. Scientists believe that Earth is about 4.6 billion years old. (Note, in this and other explanations the US billion is the number used. In the US one billion is 1000 million.) 10. What is Plate Tectonics? This is a relatively modern and widely accepted theory concerning the structure of Earth's crust. Plate tectonics proposes that the Earth's crust is made up of six major plates and several smaller ones. The plates consist of the crust including a thinnish layer of the upper mantle. These plates have moved throughout geologic time converting the very early land mass known as Pangaea into the present-day positions of continents. The North American plate extends from the middle of the Atlantic ocean to the West Coast of the United States ending at California's San Andreas Fault - an active earthquake zone. The Pacific plate is entirely under the Pacific Ocean and has no continental areas. Plate margins, where the six main plates roughly coincide, are generally zones of seismic or volcanic activity.

11. How have Earth's mountains been formed? Mountains are formed in several different ways. For example the Appalachian Mountains on the east coast of the United States are fold mountains that were pushed up as ridges on Earth's crust when the Eurasian and American tectonic plates collided. These mountains are perhaps several hundred million years old. The Sierra Nevada mountains of California were formed through a fault-block process in which large chunks of rock sank leaving other chunks standing. The Black Hills of South Dakota are dome mountains that were pushed up by a hot spot in the mantle of that region. The Rocky Mountains of the Western United States are thought to be quite young, perhaps only about 65 million years old. They are being studied by geologists to determine the method or methods by which they were formed. 12. How much of Earth's surface is covered by water or ice? Seventy-five percent of Earth's surface is covered by oceans (and most of the water is salty). About five percent of Earth's water is frozen solid and exists as glaciers covering about ten percent of the land surface. The glaciers are are generally located in high mountain ranges but there are huge ice sheets in Greenland and Antarctica.

Ten or twenty thousand years ago a large glacier or ice sheet covered most of North America. 13. What is a desert? A desert is any area of the world that receives less than one inch of annual rainfall. Quite often deserts exist in the rain-shadow of great mountain ranges. The Mojave Desert of California is one example and the Sahara Desert of Africa is another. We often equate deserts with sand dunes and of course many desert areas are sandy but deserts more commonly consist of bare, arid ground with small patches of poor vegetation. Climates as well as landforms have changed over the eras of geologic time and there is evidence that the Sahara Desert was covered by a glacier several hundred million years ago. 14. All rocks are not the same. There are three main types of rocks - igneous (fire-formed), sedimentary (water-deposited), and metamorphic (changed). This classification is based on the way in which the rocks were formed rather than on their structure or appearance. About 75 percent of rocks on land surfaces of earth are sedimentary, most of the rest is igneous and very little is metamorphic. On the other hand, most of the rock of deep ocean floors is igneous.

15. Are all igneous rocks the same? No. Igneous rocks are formed by the cooling and crystallization of magma from beneath Earth's mantle. Igneous rocks that form slowly at great depth are called plutonic and they have rather coarse structure like granite, diorite or gabbro. Igneous rock that forms from the magma extruded onto Earth's surface by volcanic eruption is called volcanic and because it cools quite rapidly it is fine-grained like basalt and sometimes glassy like obsidian. Some igneous rock is formed at shallower depths than the plutonic rocks and is called hypabyssal. Such rock is medium grained like diorite. Igneous rocks can also be classified chemically according to their silica content. Those with high (above 66 percent) silica content are called acidic, and those with low (less than 45 percent) silica content are called ultrabasic . 16. How are sedimentary rocks formed? This sort of rock is formed from sediments that have accumulated and over long periods of time have consolidated into rock.

The sediments are often fragments that have been worn away from pre-existing rocks by a mechanical process such as the abraiding action of wind, water or ice. Examples of sedimentary rock formed from such fragments are conglomerates, sandstones, and shales. Sedimentary rocks are also formed from sediments (precipitates) that are the result of chemical action. Examples are evaporites and sedimentary iron ores. Organic sedimentary rocks are formed from the remains of plants and animals. Examples are coal and limestones. 17. How are metamorphic rocks formed? This type of rock is rock that has been changed from its original form by high temperatures, pressures, or chemical action in such a way that its structure is changed. For example, a fine-grained limestone, subjected to increased pressure and temperature, can change over time into marble. Two other metamorphic rocks are slate and quartzite. Slate is a dark, smooth rock that breaks into smooth flat sheets of rock - it was once shale; quartzite is usually yellowish brown and one of the toughest rocks - was once sandstone Marble can be many colors depending upon the types of minerals that were in the particular sedimentary limestone from which it was made.

Marble is often a beautiful rock and has been a favorite material for sculptors and builders over the ages. 18. What are rocks made of? All rocks are made of chemical compounds called minerals. Most minerals are crystalline and there are several thousand know minerals each containing specific elements and having a particular molecular structure. Quartz is a mineral and is a common constituent of granite (an igneous rock) where it exists as small crystalline pieces fused together with other minerals such as feldspar and mica. Quartz, which is silicon dioxide (SiO2). can be also be found as large, colorless, transparent, six-sided crystals. Even rocks, like chalk, that are made up of the remains of once-living organisms, are made of minerals because the animal remains are fossilized and consist largely of calcium carbonate (CaC03). By the way, although chalk rock will make the familiar white marks on a blackboard, commercial blackboard chalk is made of gypsum which is calcium sulphate. 19. What mineral substances are MOST rocks made of? Most rock-forming minerals are silicates (oxides of the element silica). About 90 percent of Earth's crust is composed of silicate minerals.

Quartz is the most common form of silicon dioxide. It makes up most of the sand at the seashore, occurs as a rock in the form of sandstone, and quartzite, and is an important constituent of many other rocks such as granite and gneiss. There are many varieties of quartz and those that occur in distinct crystals are named chiefly according to their color, for example: rock crystal is clear and colorless; smoky quartz has probably darkened as a result of exposure to radioactivity; amethyst crystals are purple; citrine is a light yellow crystal; rose quartz is pale to deep pink. Other silicate rock forming minerals, such as the feldspars (which like quartz are found in most igneous rocks like granite and gneiss) are silicates of aluminum with potassium, sodium, and calcium. 20. Are some minerals harder than others? Yes. A German mineralogist, Friedrick Mohs (1773-1839) developed a very practical mineral scale of relative hardness. He listed ten minerals ranging from 1. talc (the softest) to 10, diamond (the hardest).

On Mohs' scale of hardness a mineral with a value up to 2.5 can be scratched with a fingernail, up to 4 can be scratched with a coin, and up to 6 by a knife. The minerals used by Mohs to establish his scale were: 1) talc; 2) gypsum; 3) calcite; 4) fluorite; 5) apatite; 6) orthoclase; 7) quartz; 8) topaz; 9) corundum; 10) diamond. 21. What about ruby and emerald. Are these minerals? Yes. Emerald (beryl - beryllium aluminum silicate), ruby (corundum - aluminum oxide), sapphire (corundum), opal (silica) and diamond (carbon) are all minerals and have been highly valued by people over the ages because of their rarity and their beauty. Diamond is highly valued by modern societies but historically has been valued less than the more colorful minerals. Most natural diamonds come from Africa but they are found all over the world. Small, synthetic diamonds can now be made and are used in industry where hard cutting edges are needed.

True emeralds are a deep green and a large, flawless emerald is more valuable than a diamond. The finest rubies come from Burma, and most opals come from Australia. 22. How do scientists classify/organize minerals? Minerals are grouped/organized/classified by scientists in several different ways depending upon the properties the scientists want to emphasize in different contexts. For example, minerals whose molecules contain the metal element iron (such as hematite, magnetite, siderite) can be grouped together as can be the zinc minerals such as sphalerite, smithsonite, willemite. However, a mineralogist might prefer a classification based on the type of compound and its properties and so would group minerals together that have similar chemical composition even though they contain different elements. For example a carbonates group would include both siderite (iron carbonate) and smithsonite (zinc carbonate) which would not be grouped together if they were being classified according to the metal element in the molecule (iron and zinc are both metal elements.) 23. What are Native Elements? These are chemical elements that are found in nature in their native (uncombined) state.

Most elements do not exist in nature as elements but instead are always found as compounds - each molecule being a combination of several elements. There are three groups of elements that are found in nature in their native state: 1) a few metals such as gold, silver, copper, platinum and iron; 2) a few semi-metals such as arsenic, antimony, and bismuth; and 3) a few non metals such as carbon, and sulphur. 24. What is ore? An ore is a mineral or rock from which a metal and certain other substances can be extracted for commercial purposes. Sometimes a metal is present in its native form; gold and platinum are found this way. Almost always however the metals occur as oxides, sulphides, sulphates, silicates etc. and the metal has to be extracted from its ore (separated from the other chemical elements) by chemical means. The branch of applied science that is concerned with the production of metals from their ores is metallurgy. 25. Which is the most abundant metallic element in Earth's crust? Aluminum - called aluminium in some parts of the world.

While aluminum is the most abundant metallic element in the Earth's crust it is not the most abundant 'element'. Oxygen is the most abundant element followed by silicon and then aluminum. Most of the aluminum is physically and chemically bound inside rocks that comprise Earth's crust. However in some areas (largely under tropical conditions in Australia, Guinea, Jamaica, Russia, Brazil, and Surinam) those rocks exposed at the surface have weathered and broken down into bauxite which is the chief ore of aluminum. Bauxite is a claylike amorphous substance from which aluminum can be extracted. Aluminum is a very light, very strong metal and is used in circumstances where lightness is as important as strength. 26. Are minerals much in demand for modern technology? Yes. There are at least 30 minerals that are particularly important in the U.S. for industrial/technological needs. The metals most in demand are: aluminum, copper, gold, silver, iron, tin, platinum, chromium, nicket, lead and zinc. The non-metals most in demand are: diamond, salt, limestone, sulphur (sulfur) and asbestos. Most of these exist almost everywhere but can not always be used.

This is sometimes because there are no appropriate methods of separating the desired mineral from the other minerals in the rock. In other cases the amounts present are too small. When minerals are present in situations where they can be extracted at a profit, they are called ore deposits. 27. How are minerals extracted from their ore deposits? In several different ways. For example, when tiny grains of gold are spread through a gravel deposit the gravel is poured onto a table that is coated with mercury. When the table is vibrated the gold grains work their way to the table surface and combine with the mercury. This method works because gold has a strong tendency to combine with mercury. The gold of course then has to be separated from the mercury. A different method is used to separate sulphide (sulfide) minerals from others in order to obtain their sulphur. In this case the conglomeration of minerals might be finely ground up and then vogorously mixed in a tank of water through which air is being bubbled. In this process, which is called flotation, the sulphide minerals cling to the bubbles and are collected from the froth that spills over the top of the tank. 28. Do minerals in rocks ever pose a health hazard? Sometimes.

For example serpentine is a common rock in California where it occurs abundantly in coastal mountain ranges; and there is a commercially important belt of serpentine in Quebec, Canada. Serpentine (magnesium silicate) is a metamorphic rock which contains soft, silky fibres of chrysotile asbestos. Asbestos fibres can withstand fire and insulate against heat and they are used in automobile brake linings, heat resistant tapes, cloth and insulating materials. A health hazard can occur during the asbestos extraction process and aslo when the serpentine rock is crushed for use in road construction. Asbestos fibres in the air are breathed into the lungs. This is a problem because asbestos fibres can work their way into tissues and cells. This can cause asbestosis which is a chronic lung disease. Certain types of asbestos are carcinogenic in that they cause lung and other cancers. 29. What is geologic time? Geologic time is a time scale of units of relative (not absolute) time in chronological order from the time of Earth's early beginnings to the present day. The time periods are called eras, periods and epochs.

We call the earliest era the Paleozoic Era and it includes several periods, the earliest of which we call the Cambrian Period. The most recent era is called the Cenozoic Era which includes the time following the extinction of the dinosaurs and the much more recent time of the appearance of modern man. Geologists interested in this aspect of Earth Science have the intriguing task of determining which rocks are older or younger than others - and by how much! - and determining approximate dates for geologic events such as, for example, the various Ice Ages. 30. When was modern geology born? Many ancient scholars observed and developed theories about the development and structure of Earth, and over the centuries the interest and body of knowledge increased but it was not until the 18th century that geological science really matured. The father of modern geology is generally agreed to have been James Hutton (1726-1797), a Scottish medical man and farmer who was very interested in the history of the Earth and in the processes that modify the Earth's surface. Before Hutton's time most geologists believed Earth was only a few thousand years old. They believed that Earth's physical features were the result of catastrophic events.

Hutton's great contribution was the formulation of the theory that natural agents now at work on and within the Earth have operated with general uniformity throughout immensely long periods of time. This is an underlying tenet of modern geology. ~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^ What would you see on a journey to the centre of the Earth? | New Humanist Our content is free to read but not free to produce. If you can help support us today. Donate What would you see on a journey to the centre of the Earth? Crystals as big as cities and a sea of liquid metal are just some of the wonders you would see. – By David Whitehouse – Wednesday, 17th June 2015 This article is a preview from the Summer 2015 edition of New Humanist. You can find out more and subscribe here .

If you could dig a tunnel right through the Earth you could theoretically free-fall to the centre of our planet in just eighteen minutes. You can’t, of course: such a tunnel would have to tolerate temperatures as high as the surface of the Sun and pressures three and a half million times greater than those at the Earth’s surface. No material yet developed could withstand those conditions. The deepest we have ever drilled is just over 12km. But just suppose you could build it, what would you find? Much of what we have learnt about the inside of our world, especially its innermost regions, we have discovered only recently. So, bold traveller, this is what you would encounter on your imaginary journey. The Crust It will take less than a minute to fall through the Earth’s crust – its outermost hard shell made of the lighter rocks. The skin of our planet is only 35km thick and a mere 1 per cent of the Earth’s mass. The crust is composed of two types. The continental crust is made of the lightest rock such as granite and it forms the continents that overlay denser rocks.

The continents have the oldest rocks found on the surface of the Earth and the record holder is a greenish, finely grained rock 4.4 billion years old and found on the shore of Hudson Bay, Canada. These rocks solidified soon after the Moon was formed, and if you were there at that time you could have looked up from them and seen active volcanoes glowing on the Moon. Studies of the earliest rocks indicate that the early Earth was not hot and hellish for very long but cooled swiftly and had extensive oceans almost from the start. Everything mankind has ever built is made from metals and minerals mined from the crust with the exception of diamonds – they come to us from much deeper. The so-called oceanic crust is much younger than the continents and is constantly being formed at mid-ocean ridges like that seen bisecting the Atlantic, from where it spreads as slabs of moving basalt. This is the driving force behind the movements of the continents. On the sea floor these slabs often gather sediments that can pile up in a thick layer.

But its life at the Earth’s surface is a relatively brief one, no more than 200 million years, for these rocks so recently solidified from the molten Earth below will soon rejoin it, bringing cold from the surface, sediments and great changes. Being heavier than continental crust, oceanic sea-floor slabs are pushed back into the Earth under the continents in a process called subduction. The Mantle Like you, the slab is heading through the crust into the mantle – the largest region of our planet, comprising about 82 per cent of its volume and 65 per cent of its mass. The mantle is where the archaeology of our planet is stored as well as newly recognised ancient structures and processes that may be necessary for life to exist on the surface. Indeed, scientists are beginning to realise that the conditions for life are created as much from below as they are by the Sun above. As the slab descends it comes under extreme pressure and, being colder than the rock it is plunging into, it is brittle. Consequently it fractures and slips, causing the most powerful earthquakes we ever experience.

Japan – the world’s most earthquake-prone nation – sits above where the Pacific oceanic plate is subducting beneath the Asian continental crust. As the slab gets deeper the temperature and pressure increase to levels that it cannot withstand. It is heated so that the rock flows like plastic, meaning that below a few hundred km the earthquakes cease. Minerals containing water break down and release fluid. The water seeps out and rises and reaches the mantle rocks above it. There it causes the melting temperature of those rocks to lower, sometimes by as much as 400 degrees. This rock, now much less viscous, makes its way to the surface. That is why a hundred or so kilometres beyond a subduction zone there are arcs of new volcanoes. The mantle – and this is generally true of the entire Earth – is mainly composed of four elements: oxygen, silicon, magnesium and iron. Their atoms link to form latticework structures that yield under pressure to form ever-closer arrangements. For many years only the mantle’s upper section was considered interesting. Below that the increasing pressure and heat were supposed to have squeezed the rocks so mightily as to erase any structures.

The result would be a uniform mass of rock reaching downwards until the dramatic boundary with the outer core is reached. We now know that the mantle is a more active place than we had thought. Although it’s made of solid rock, it does move slowly with material descending to and rising from its deepest layers. Some scientists believe there are plumes of rock making their way through the mantle heading for the surface and that every few hundred million years vast amounts of lava are disgorged at the surface causing mass extinctions. Tuzo and Jason By the sophisticated analysis of shock waves from earthquakes that bounce around in the interior, scientists have found four giant structures at the base of the mantle. There are two regions that seem to be relatively cool and sinking regions, one under the western edge of the Americas and the other under southern Eurasia. There are also two large and mysterious regions of higher than average density lying beneath Africa and the Pacific. The African region towers above the core–mantle boundary by well over a thousand km and the Pacific region is only a little lower. Together they cover half of the world. They have been given names, Tuzo and Jason, after the pioneering earth scientists W. Jason Morgan and Tuzo Wilson.

They are truly giant structures, each 15,000 km across, and are considered by some scientists to be our planet’s underground continents. Research suggests they are ancient and are likely to have formed 4.4 billion years ago when the Earth was young. Recent data indicate that the edges of Tuzo and Jason appear sharp and scientists speculate that they deflect material upwards, for what goes down to the lower mantle can come back up again billions of years later. Liquid Metal On your journey to the heart of our planet you will reach the base of the lower mantle in about eight minutes. You you must then prepare for the biggest shock of your journey. Nowhere in or on our planet is there such a dramatic change of scenery as the one you are about to experience. Suddenly, at a depth of 2,890 km, you burst through the rocky part of the Earth into a sea of liquid metal five thousand degrees hot. This is the outer core, occupying about 10 per cent of the Earth’s volume and 27 per cent of its mass. That’s about the size of the planet Mars.

Imagine if our capsule could pause just above the boundary in the white-hot rock of the lower mantle, and then move perhaps just a few tens of metres into the liquid metal of the outer core. It would be like diving into an ocean from a slightly curving wall of rock. You have left behind the rocky regions of our planet and now, for the next 2,000 km, you are a submariner in a sunless sea of slow-motion currents, storms and cyclones of liquid metal riven with magnetic and electrical fields. If you donned super-protective gloves you could run your hands through it like water. Its influence extends further than the Earth itself, for out of its motions emerges our great protector – the Earth’s magnetic field. Without our magnetic protection life on the surface would not be possible, for it shields us from harmful particles from the Sun and deep space. Mars lacks such a defence and has lost most of its atmosphere. Consequently no life can exist on its exposed surface. Every once in a while something happens to the currents in the outer core. They seem to become chaotic and no longer generate as strong a magnetic field. On the surface the magnetic strength declines to about ten per cent of what it was.

Over thousands of years it increases as the outer core recovers from the disturbance. But the magnetic poles have reversed: north becomes south and vice versa. It has happened many times in the past and life has survived. There is no indication it is happening now, though some day it will be something humanity will live through. The Crystal Core After eight more minutes of descent you reach the greatest mystery of our planet: a super-dense ball of solid iron and nickel as hot as the surface of the Sun. It is only solid because of the pressure of over a million times that of the surface. This is the crystal core, only about half a per cent of our planet’s volume, a little smaller than the Moon, but nearly 2 per cent of our planet’s mass, and one of the strangest objects ever encountered by science. Imagine the volume of water in all of the world’s oceans and multiply it by five and you have the volume of the inner core of the Earth. It is difficult to gather information about the inner core. It presents a small target for seismologists. Few seismic waves reach it, let alone make it back to the surface to reveal what they have travelled through.

New techniques are being developed to pick out the faint and elusive waves that traverse the inner core. With the right processing it’s possible to identify the faint echoes from the inner core, and the more we analyse them the stranger it becomes. This iron-clad world is floating within a metal sea and is not obviously tied to anything above. There are indications that it rotates at a slightly different rate from the rest of the Earth, and is also offset from the very centre of the Earth by many kilometres. Some believe that the boundary between the inner and the outer core is undulating and mushy, with the iron crystals growing dendritically; reaching out like giant iron trees with a kind of iron mush between them. Others believe the topmost few hundred kilometres of the inner core consist of small crystals of iron, but inside the inner core they may merge, losing their individual identity as they become giant crystals aligned with the Earth’s overall magnetic field. These crystals are marvels, unseen wonders of the solar system. If you could sail past them they might remind you of a geological structure similar to the basalt columns of the Giant’s Causeway in County Antrim, Northern Ireland, though thousands of times longer and wider.

Single iron crystals the width of a city, stretching the distance from London to Birmingham! But the biggest mystery is that the inner core is a newcomer. The latest calculations indicate it only appeared between 500 and 1,000 million years ago, having grown steadily from a single crystal of iron. It continues to grow at half a millimetre a year and in a billion years it may even switch off the Earth’s magnetic field. We will never reach the centre of the Earth. The conditions are so harsh – six thousand degrees Centigrade and a pressure of three and a half million atmospheres – that no probe will ever get there. Perhaps it will always be an enigma. A scientist involved in inner-core research told me, “Everything is getting increasingly complex as we get more data. We are seeing more and more complexity, and the more we do the less we understand.” David Whitehouse’s book, Journey to the Centre of the Earth, is published by Weidenfeld & Nicolson This article was brought to you by New Humanist, a quarterly journal of ideas, science and culture. To support our journalism, please subscribe .

A simple way to support New Humanist, share this article with friends UCSB Science Line UCSB Science Line How do scientists known what the center of the earth is? Answer 1: Geologists hypothesize that the Earth's center is composed of predominantly iron. The outer portion of the center, known as the "outer core", is thought to be liquid iron. While the interior portion of the center, known as the "inner core" is thought to be solid iron. This hypothesis is based upon two important observations. First, energy waves known as "shear waves" do not travel through the outer core. Secondly the Earth has a magnetic field. A Geologist uses seismometers to measure shear waves caused by earthquakes. Once shear waves hit the outer core the wave 'disappears' and once it hits the inner core it reappears. Think of a shearing force as the force needed to rub your hands together. This sliding or shearing force cannot travel through liquids because liquids do not resist shear waves. Shear waves do travel through solids however. Therefore since we see shear waves disappear at the outer core and reappear at the inner core geologists hypothesize that the outer core is liquid and the inner core is solid.

Regarding the Earth's magnetic field, the Earth exhibits a magnetic field that reaches above our atmosphere. For a magnetic field to exist, either a permanent magnet must be in the Earth's interior, or ionized molecules are moving in a liquid medium in the Earth's interior. A permanent magnet cannot exist in the Earth's interior due to the high temperatures deep in the Earth. Therefore the magnetic field is thought to be produced by ionized iron moving about in the Earth's liquid outer core. You pose an excellent question. The composition of Earth's interior is critical to all life on Earth. If Earth did not have a liquid outer core the Earth would not have its magnetic field. Earth's magnetic field protects us from cosmic and solar energy that would prevent life as we know it. Answer 2: The main evidence for the structure of the Earth comes from studying waves generated during an earthquake. When stress builds up and rocks at depth break, then elastic waves, just like sound waves are sent from the point of rupture. These wavesspeed out and move through the earth. When we put seismometers on the surface of the Earth, these waves can be measured.

In particular, if we have enough seismic stations and the quake is big enough, then we can take all the travel time information and invert it. That means we can obtain a graph that shows the density of rock as a function of depth. Once we know the density we can estimate what the actual materials are. We find that the earth has a small inner core made out of iron surrounded by a larger outer liquid iron core and THAT is surrounded by a rocky mantle and rocky crust. If you want to know more about this, you should study seismology. A good place to START is Answer 3: Good question! Earthquakes (and nuclear tests) cause seismic waves that travel through the Earth's interior, but these waves travel at different speeds depending on what kind of material they are traveling through. By having listening posts scattered around the planet, we can deduce what layers the Earth's interior to possess based on how long it takes for these waves to reach the listening post from the earthquake that produced them. We then can match the speed of these seismic waves with materials we are familiar with and identify what the Earth's interior is made of - at least in theory.

We also know how dense the Earth is by how strong the Earth's gravity is, and we know that the Earth's average density is about 5 grams per cubic centimeter. Rocks have a density less than 3 grams per cubic centimeter, which means that much of the Earth's core must be made of denser stuff in order for the whole planet to average out to 5. Metals have a density around 10 grams per cubic centimeter, so we speculate that the Earth has a metal core. The speed of seismic waves through the Earth's very center, also suggest speeds similar to what you would get in very hot metal. So what is this metal? Well, based on how much of what kinds of metals there are in the solar system, we guess that it's mostly a mix of iron and nickel - but I don't think we actually know. Answer 4: Scientists know about the Earths interior from observations of Earths gravity, seismic waves that travel through the Earth, and the Earths magnetic field, as well as from comparison with the chemical composition of meteorites and from experiments that simulate conditions at the center of the Earth.

The first estimate of the density of the Earths core was made by Isaac Newton over 300 years ago. Because the strength of a planets gravitational field depends on its density, Newton was able to use his observations of the Earth gravity to calculate that the average density of the Earth was more than twice that of rocks at the surface. Thus, the interior of the Earth must be much denser than the rocks near the surface. More detailed knowledge of the structure of the Earths interior comes from observations of seismic waves. When an earthquake happens, it releases energy that travels through the Earth in all directions as seismic waves. Two kinds of waves created by earthquakes are compressional (P) waves and shear (S) waves. Both of these waves can travel through solids, but S waves cannot travel through liquids. Scientists figured out that the outer core must be liquid because S waves do not pass through it, but P waves do. The behavior of P and S waves also indicates that the inner core is solid. The speed of seismic waves also depends on the density of the material through which they are traveling. Thus by observing many seismic waves from many earthquakes all over the world, scientists have been able to work out the density of different parts of the Earth (i.e.

the core, mantle, and crust). So, how do we know what the dense material at the Earths core actually is? Scientists believe that the overall chemical composition of the Earth is very similar to a kind of meteorite called chondrites, which formed at the same time the Earth was formed. We know a lot about the composition of the Earths crust and mantle, because we can observe those rocks that have been brought to the surface by geologic processes. By comparing the composition of rocks from the Earths crust and mantle to the composition of chondrites, we can see what elements are missing, and therefore must be found in the core. Theories about how Earths magnetic field is formed, as well as experiments done at high temperature and pressures give clues as to the actual composition of the Earths core. Based on all these theories and observations, scientists know that the Earths core is mostly iron with some nickel and lighter elements such as oxygen or sulfur. Click Here to return to the search form. Copyright © 2015 The Regents of the University of California, All Rights Reserved.

Question: Which two elements would you find at the centre of the Earth?
Answer: Iron and Nickel